The invention relates to a hydraulic steering system comprising a first and a second steering member and a vehicle comprising a hydraulic steering system.
Work machines, also known as construction equipment or construction vehicles, as for instance excavators or wheel loaders are vehicles designed for and used in rough off-road surroundings where trucks or passenger cars are either inoperative or, if operated in such surroundings, would probably be damaged when exposed to these rough conditions.
It is known in the art to provide such work machines with a steering wheel and an additional joystick steering which provides an ON/OFF control of the steering direction. Typically, the joystick is used to control a tool, e.g. an excavator bucket, and a steering device is mounted on the joystick which allows steering of the excavator wheels.
U.S. Pat. No. 6,408,977 B1 discloses a method to steer front wheels of a mobile work machine. A normal steering wheel is coupled to an additional joystick steering device. Steering by the joystick steering device is not possible while the steering wheel is being turned. A pressure sensor gives an output depending on the usage of the steering wheel and this signal inhibits the usage of the joystick steering device. Further, if the steering wheel is not turned, but the working machine is moving straight ahead with a high speed above a certain tolerable threshold (for instance in a travel mode on a road) the usage of the joystick steering is also inhibited.
US 2007/0209356 A1 discloses a vehicle comprising a hydraulic system comprising a first steering valve associated with a first user input and a second steering valve associated with a second user input. The first user input is a steering wheel and the second user input is a joystick. The hydraulic system is configured to give priority to the first input over the second input by ways of a priority valve. The hydraulic system includes a single hydraulic circuit. A control circuit is arranged between the priority valve and two solenoid control valves which provide a pilot pressure for the second steering valve.
It is desirable to provide a hydraulic steering system for a vehicle which increases the comfort for the operator and the safety of operation. It is also desirable to provide an adequate method for operating a hydraulic steering system of a vehicle in a comfortable manner.
According to a first aspect of the invention, a hydraulic steering system, for a vehicle is proposed, comprising a first steering member and a second steering member, wherein the first steering member is operationally connected to a first steering valve unit comprising a first steering valve, and wherein a priority valve controls a flow of a hydraulic fluid to one or more steering cylinders which provide for steering of the vehicle in a desired steering direction, wherein the priority valve gives priority to steering with the first steering member over the second steering member in case the first steering member is operated.
A second steering valve is provided functionally operable in series with a proportional flow control valve, wherein the second steering valve and/or the proportional flow control valve are/is controlled by the second steering member. The control can favourably be achieved by an electrical signal or by a hydraulic pilot signal, or by an electromagnetic signal.
Favourably, the second steering valve is controlled by a first control signal and the proportional flow control valve is controlled by a second control signal from an electric output signal of the second steering member corresponding to an actuation of the second steering member. The output of the second steering member can be used as input for an Electronic Control Unit that will calculate the two control signals to be sent to the proportional flow valve and the second steering valve.
Particularly, the present invention can be employed, according to an aspect thereof, to steer the ground engagement elements of a work machine, for instance the front or back ground engagement elements or all such elements. The ground engagement elements can be wheels, tracks and the like. It may be possible that both the front and the back wheels of an excavator can be steered simultaneously in a coordinated-way when turning the steering wheel or operating the joystick. Further, work machines with tracks are usually steered differently than work machines with wheels in that for instance the left side track is moving counter-clockwise while the right side track is moving clockwise (or vice versa) or in that only one of the two tracks is moving while the other track is blocked. The mobile work machine can particularly be an excavator.
The first steering member can be e.g. a steering wheel and the second steering member can bean actuator on a joystick. The joystick steering can be arranged additional to the normal steering actuated by the first-steering member, e.g. a steering wheel. The first steering-member, however, can also be a joystick, a sliding actuator or the like. The present invention provides, according to an aspect thereof, a better control and comfort for the operator as well as improved safety and controllability of the steering function of the work machine.
Preferably, the first control signal can be an ON/OFF signal which can be, applied to the second steering valve responsive to the second steering member for selecting the steering direction. Particularly, the valve can comprise 3 positions, “left on”, “right on”, and “left and right off”, which is expressed as “LEFT—ON/RIGHT—ON/OFF” describing in short form the three possible states of the valve “LEFT—ON” or “RIGHT—ON” or OFF”. The second control signal can be a Pulse-Width-Modulation (“PWM”) signal, or Pulse-Frequency-Modulation (“PFM”) or Pulse-Step-Modulation (“PSM”) which can be applied to the proportional flow control valve responsive to the second steering member for controlling a steering characteristic.
Favourably, the two valves can be controlled by two different electrical signals generated from a common signal source, i.e. the actuation of the second steering member. The second steering member can be a thumb slider, a thumb wheel, a hat switch (also called “mini joystick”) which is operable with one's thumb. The second steering member may include a self-centering feature that brings the second steering member back in a neutral default position when not in operation or not being operated.
According to a further development of an aspect of the invention, the second steering valve and the proportional flow control valve can be connected in series with respect of the flow of the hydraulic fluid. The proportional flow control valve can be upstream to the second steering valve with respect to the hydraulic pump which provides the hydraulic pressure in the hydraulic system.
Particularly, the second steering valve can be a so called “4/3 way” valve, the expression “4/3 way” denoting a valve which has 4 fluid ports and 3 positions as known in the art. The second steering valve can be controlled with the help of one or more solenoids which switch the valve between the various positions.
The proportional flow control valve controls the amount of hydraulic fluid flow which is sent to the steering cylinders of the vehicle and can be controlled by an Electrical Control Unit (“ECU”), thus providing various possibilities of control characteristic, such as a smooth, aggressive, linear, nonlinear and the like steering movement as response to an actuation of the second steering member. By way of example, if the second steering member is a thumb slider the steering characteristic can react smoothly or aggressive or linear etc. to a sliding deflection of the thumb slider.
The combination of the second steering valve and the proportional flow control valve and their series connection yields an additional safety feature. As the two valves are controlled by different signal types, e.g. an ON/OFF signal in case of the second steering valve and for instance a PWM signal in case of the proportional flow control valve, the risk that a dangerous failure mode will have effect will be considerably decreased with the preferred series connection and electrical control of the second steering valve and the proportional flow control valve. Further, by designing the valves and their control logic in a fail-safe mode this risk will be decreased even further in that such design would block the hydraulic flow through a valve in case of failure of the valve concerned.
The steering valve and the proportional flow control valve can favourably form a valve unit which is connected in parallel to the first steering valve unit responsive to the first steering member. However, it is also possible to arrange them separately in different valve units. The important thing is that these two valves are operated in such a way that they are functionally operable in series with each other irrespective where they are physically located within the hydraulic steering circuit in the vehicle.
The second steering member can favourably be integrated in a joystick. Preferably, the second steering member can favourably be one of a thumb slider, thumb wheel, a hat switch being operable with one's thumb, arranged on an upper portion of the joystick. The joystick can be used to control a tool connected to the vehicle, e.g. excavator bucket. Thus, the steering can be controlled very sensibly by the operator while the operator is operating the tool of the vehicle.
A first pressure sensor can be connected to a hydraulic fluid load line between the priority valve and the first steering valve unit, wherein the pressure sensor output is coupled to the second steering member so that activation of the second steering member is controllable depending on an output signal of the pressure sensor.
Additionally, or alternatively, another second pressure sensor is connected to a hydraulic fluid load line between the priority valve and the first steering valve unit, wherein the pressure sensor output is coupled to the second steering member so that control signals applied to the second steering valve and/or the proportional flow control valve can be modulated according to the actual pressure signal measured by that other pressure sensor.
The first pressure sensor can particularly output a signal which can be an analogue input to the ECU controlling the priority valve. The pressure in the hydraulic fluid load line increases due to a movement, of the first steering member. If the first steering member is not in use, the pressure in the hydraulic fluid load line is low, e.g. on hydraulic tank pressure level. Therefore, the pressure signal of the first pressure sensor can be used to disengage the electrical signals to the second steering valve and to the proportional flow control valve when the first steering member is operated and the pressure signal of the first pressure sensor is changed accordingly, thus providing an additional electrical priority for the first steering member. This can be implemented as additional safety feature in a control routine of the vehicle.
Mechanical blocking or inhibition of the second steering member when the first steering member is operated can be provided as well but is usually not necessary.
Further, the electrical inhibition of the second steering unit is also made when the vehicle is in a mode of operation in which it is enabled to move and in which it could travel with a speed exceeding a certain (predefined) speed limit, for instance when travelling on a public road.
One advantage of an aspect of the invention is that the system is flexible and can operate or change its function without the necessity to make massive hardware design changes.
Based on virtually the same-hardware, i.e. the second steering valve unit, the invention, according to an aspect thereof, provides two different operational modes. As already described, the second steering member and the second steering valve unit can be employed to steer the ground engagement elements of a work when the first steering member is not engaged, according to the first operational mode.
In the second operational mode of an aspect of the invention; the second steering valve acts as a kind of servo unit for the first steering member. In this preferred alternative way of usage of the second steering valve unit, the pressure signal of the first and/or second pressure sensor can be used to modulate the electrical control signal to the second steering valve and the proportional flow control valve to change the reaction of the steering cylinder by adding a part of the total bypass flow of hydraulic fluid to the normal hydraulic fluid flow that the priority valve would give for the same pressure value on the hydraulic fluid load line. This feature can be considered as an automatic modulation (servo function) of the steering movement which modifies the necessary number of steering wheel turns to move the steering cylinder from one end to the other end. In case the first or second pressure sensor is used for this purpose, the second steering valve unit will not be, disengaged when the first steering member is operated. In this case it is advantageous to include a steering direction sensor which detects the steering direction in which the first steering unit is moved, i.e. if the vehicle is to be steered to the right hand side or the left hand side.
In order to distinguish the two alternative operational modes of, aspects of the invention, this alternative second operational mode of an aspect of the invention can be made selectable by the operator by e.g. an actuator such as a switch, a lever or the like, or a vehicle control unit can simply recognize the presence of the steering direction sensor or if the sensor is in its active operational state. Such a steering direction sensor can be present but is not necessary for the first above-described alternative operational-mode of an aspect of the invention. Advantageously, the second steering unit can be used in these two alternative ways even with virtually the same hardware.
Favourably, an ECU is provided for controlling the second steering valve and/or the proportional flow control valve.
According to another aspect of the invention, a method for controlling a hydraulic steering for a vehicle is proposed, comprising a first steering member operationally connected to a first steering valve unit and a second steering member, wherein a priority valve controls a flow of a hydraulic fluid to one or more steering cylinders which provide for steering the vehicle in a desired steering direction, wherein the priority valve gives priority to steering with the first steering member compared to the second steering member in case the first steering member is operated.
A second steering valve is provided functionally operable in series with a proportional flow control valve, wherein the second steering valve and/or the proportional flow control valve are/is controlled by the second steering member.
The control can favourably be achieved by an electrical signal or by a hydraulic pilot signal, or by an electromagnetic signal.
Favourably, a first control signal and a second control signal are extracted from an electric output, signal related to an actuation of the second steering member, wherein the first control signal controls the second steering valve and the second control signal controls the proportional flow control valve, wherein both the second steering valve and the proportional flow control valve are responsive to the second steering member.
Particularly, the present invention relates, according to an aspect thereof, to a method to steer the front or back wheels of a wheel excavator or (in a coordinated manner known in the art) both the front and back wheels via an actuator on a joystick. This joystick steering is additional to the normal steering actuated by the steering wheel.
The second steering valve can be controlled by an ON/OFF signal (particularly LEFT—ON/RIGHT—ON/OFF) for steering the vehicle in a desired steering direction. The proportional flow control valve can be controlled for instance by a Pulse-Width-Modulation (“PWM”) signal for controlling a steering characteristic. Other types of modulation signals as for instance Pulse-Frequency-Modulation (“PFM”) or Pulse-Step-Modulation (“PSM”) signals can be used as well.
According to the first operational mode of an aspect of the invention, if a steering movement of the first steering member is detected, the pressure in a hydraulic fluid load line between the first steering valve and the priority valve increases. According to this increasing pressure, the priority valve directs the flow of a part of the hydraulic flow as a function of at least one pressure signal in the hydraulic fluid load line to the first steering valve, thereby enabling the steering of for instance the wheels of the excavator by operating the first steering member. Preferably, the steering priority valve is a proportional valve. The output of the valve is proportional to the need input from the first steering valve.
The second steering valve unit is electrically switched off if the pressure in the hydraulic fluid, load line is equal to or above a predetermined pressure limit.
Favourably, it can be ensured by these two features that the steering with the first steering member is prioritized.
If no steering action is performed with the first steering member, but a steering action of the second steering member is detected, the proportional flow control valve of the second steering valve unit can be triggered, wherein simultaneously the second steering valve is controlled to provide a desired steering direction indicated by a steering action of the second steering member.
In a preferred embodiment of an aspect of the inventions, the steering of the vehicle in the second operational mode and the speed of the vehicle are coupled with each other. Depending on the starting point, if the second operational mode is selected in a low gear of the vehicle transmission for which the use of the second operational mode is allowed, it is not possible to switch to a higher gear for which the use of the second operational mode is not allowed thereby effectively limiting the possible travel speed of the vehicle whereas if the vehicle travels with a speed above a certain speed limit (or if a gear of the vehicle transmission is engaged that allows such high speed), the second operational mode cannot be selected. Therefore, the travel speed of the vehicle is automatically limited to speed values below a certain upper speed limit when selecting a steering mode employing the second steering member. Thus it is not possible choose a travel speed of the vehicle which is higher than a certain speed limit simultaneously with the steering mode employing the second steering member.
Particularly, the safety of the vehicle can be improved if at least one of the following is given                a high travel speed of the vehicle above a speed threshold value prevents selecting a steering mode employing the second steering member;        selecting a steering mode-employing the second steering member prevents a high travel speed of the vehicle-above a speed threshold value;        selecting a gear which is not compatible with the steering mode employing the second steering member prevents selecting a steering mode employing the second steering member;        selecting a steering mode employing the second steering member prevents selecting a gear which is not compatible with the steering mode employing the second steering member.        
Favourably, the invention can be employed, according to an aspect thereof, in any vehicle or construction equipment comprising a hydraulic steering. The invention, according to an aspect thereof, is particularly useful for work machines such as wheel excavators.
The present invention is particularly applicable, according to an aspect thereof, for vehicles, particularly construction equipment, where (1) either the front or the back wheels are turned, or in a coordinated way even both the front and back wheels are turned like in wheeled excavators or the left hand side and right hand side tracks of crawler excavators etc., or where (2) the vehicle or construction equipment is articulated like a wheel loader or articulated hauler having wheels which can't be steered.
The present invention provides, according to an aspect thereof, the operator a higher comfort to control the mobile work machine during operation by providing the operator a steering control (i) which can be easily handled by the operator, (ii) which enables a smooth and proportional control of the steering speed, (iii) which increases the safety by providing diagnostic possibility for the device giving priority to steering, and (iv) which provides the option to implement additional features and functionalities by using an ECU and electronic control of a proportional hydraulic flow control valve additionally to a second steering valve.